You might consider providing a bit more information considering that each model may have a different answer. Coatings inside etc etc.
I have heard of the fuel spilling out of the fill neck if you fold the wings with fuel in them, ill advised as I understand it. I have myself experienced fuel flowing directly out of the vent tube on the cap when I deflated my 22" tire on one side and the fuel transfered over from the high wing right out onto the hangar floor. It flows really well I assure you. I dont fold my wings. Your question might be concern over integrity? If you have the Kitfox manual you should easily find the suggested methods for verifying that the tank is sound. Pressure test essentially. $.02
Good luck.